1. Field of the invention
The present invention relates to a variable power optical system used in a copying machine.
2. Description of Related Art
As is well known, it is preferable that a variable power optical system for a copying machine, which is mainly used at an actual size image or full size image (magnification is approximately 1) is in a symmetrical arrangement because it is not necessary to take into account a transverse chromatic aberration or distortion. In a symmetrical arrangement, a curvature of field or a coma (comatic aberration) tends to occur. To prevent this, it is known that the symmetrical arrangement can be changed to an asymmetrical arrangement upon varying the magnifying power to thereby correct the curvature of the field or the coma, as disclosed for example in Japanese Unexamined Patent Publication Nos. 1-123210 or 4-296710. However, depending on the degree of asymmetry, a transverse chromatic aberration or a coma is caused. This results in a decrease in the contrast or deterioration of the image quality.